1. Field of the Invention
The present invention relates to an LED (light emitting diode) driver, especially to an LED driver capable of regulating its power dissipation when driving an LED module.
2. Description of the Related Art
For general LED lighting apparatuses which have a driver for LEDs, as the lifetime of an LED is extremely long, and often longer than that of the driver, the operating lifetime of the LED lighting apparatuses is therefore determined by the driver rather than by the LEDs. In other words, if the power dissipation of the driver is not well controlled, the operating lifetime of an LED lighting apparatus can be much shorter than expected.
Please refer to FIG. 1, which illustrates a circuit diagram of a prior art LED lighting apparatus. As illustrated in FIG. 1, the prior art LED lighting apparatus includes an LED module 100 and a driver circuit 110.
The LED module 100 has a first end coupled with a line voltage VIN, which is a processed voltage from an AC line and can be a DC voltage or a full-wave rectified voltage, and a second end coupled with the driver circuit 110.
The driver circuit 110 has a third end coupled with the second end, a fourth end coupled with a reference voltage VREF, and a fifth end coupled to a ground. The driver circuit 110 includes an amplifier 111, an NMOS (N type metal oxide semiconductor) transistor 112, and a resistor 113. The amplifier 111 has a positive input end, a negative input end, and an output end, the positive input end being coupled with the fourth end. The NMOS transistor 112 has a drain, a gate, and a source, the drain being coupled with the third end, the gate being coupled with the output end of the amplifier 111, and the source being coupled with the negative input end of the amplifier 111. The resistor 113 has one end coupled with the source, and another end coupled with the fifth end.
When in operation, the LED module 100 has a first voltage V1 between the first end and the second end, and the driver circuit 110 has a second voltage V2 between the third end and the fifth end, wherein VIN=V1+V2, and the driver circuit 110 will force the voltage at the negative input end of the amplifier 111 to follow the reference voltage VREF, so as to result in a constant current IO=VREF/(the resistance of the resistor 113). However, when VIN increases due to a variation of the AC line, or V1 decreases due to a temperature increment, V2 will increase and the power dissipation in the driver circuit 110 will increase accordingly, which may cause an overheat and may thereby damage the driver circuit 110.
To solve the foregoing problem, a novel LED driver is needed.